A varistor is an electronic component having a “diode-like” nonlinear current-voltage characteristic. The name “varistor” is a combination of the two words “variable” and “resistor”. However, a varistor may also be referred to as a voltage-dependent resistor (VDR). As the names imply, a varistor has a resistance or impedance that varies as a function of voltage. Generally, at low voltages a varistor may exhibit a high resistance or impedance, and at high voltages the varistor may exhibit a low resistance or impedance. Because of this behavior, varistors are often used to protect circuits against excessive transient voltages, commonly known as surges. For example, one or more varistors may be incorporated into a circuit in a particular manner, such that, when an overvoltage or surge occurs, the one or more varistors redirect any current created by the high voltage away from sensitive components.
One common type of varistor is the metal-oxide varistor (MOV). An MOV is generally composed of metal oxide materials (e.g., zinc, bismuth, cobalt, manganese, etc.) inserted between two conductive metallic electrode plates. Many surge protection devices (SPDs) include internal MOV suppression components to protect against transient overvoltages resulting from a lightning strike or other event. In practice, each plate of an MOV may be connected to a different conductor. Depending on the desired system configuration and surge protection, each plate may be connected to a particular electrical pole, phase, neutral, or ground. By way of example, a three phase SPD may include line-to-neutral (“L-N”) MOVs connected phase A to neutral, phase B to neutral, and/or phase C to neutral; neutral-to-ground (“N-G”) MOVs connected neutral to ground; line-to-ground (“L-G”) MOVs connected phase A to ground, phase B to ground, and/or phase C to ground; and/or line-to-line (“L-L”) MOVs connected phase A to phase B, phase B to phase C, and/or phase C to phase A. Each configuration is commonly referred to as a mode of protection.
During normal operating voltages, an MOV exhibits high impedance with low leakage current. Thus, during normal operating voltages, the MOV provides electrical isolation between two conductors to which the electrode plates are connected. On the other hand, during an overvoltage condition, the MOV rapidly decreases impedance, which permits the overvoltage to flow through the MOV as current. Thus, during an overvoltage condition, the MOV redirects surge energy away from one conductor to another conductor, thereby equalizing voltage and protecting sensitive loads. By definition, transient overvoltages or surges are momentary, short duration events. After the overvoltage concludes, the MOV resets itself back to a high impedance state and awaits future surges.
When sized appropriately, MOVs are generally able to control hundreds or thousands of surges. In some instances, multiple MOVs may be installed in electrically-parallel configurations in the same mode of protection. This may offer various benefits, including but not limited to: increased surge capacity, lower let-through voltage performance, increased robustness and redundancy, etc.